RADEON Xinerama
It's a trick to get a radeon card with two outputs to work with xinerama. It may be the same for other two-output cards.
Apparently we had a version of the radeon driver that didn't yet support MergedFB, so don't ask about that. We're too lazy to do something like download the new driver. So we tried to get xinerama working.
In the end, this is what we had to do, and it's a bit counterintuitive. You
need two Screen sections, as is normal for a Xinerama setup. A Screen section
maps a monitor to a video card. They use the same card, right, so they ought to
use the same Device section. Well, nope. You need 2 device sections, and
not only that but you have to specify the BusID on both, even though it's the
same BusID.
So basically, set up xinerama per the xinerama howto and pretend like you do have two cards - they'll just both be on the same busid.